This invention relates to systems for safe-guarding operators of machine tools and more particularly to systems of this type which establish a radiant energy screen in front of a cutting zone or the like to detect the presence of some portion of the operator's body in the immediate vicinity of the cutting zone.
A variety of protective mechanisms have heretofore been utilized to safeguard the operators of large powered machine tools which have a work zone at which cutting operations, work stock deformation or the like takes place. In some instances the protective mechanisms simply create a physical barrier around the work zone at critical stages of machine tool operation. Another type of protective machanism does not impose any actual barrier between the operator and the work zone but acts instead to detect the presence of any sizable object, such as the operator's hand, for example, in the vicinity of the work zone. In many cases, these detector mechanisms react to detection of such an object by stopping operation of the machine tool.
In one highly advantageous system of this kind radiant energy such as infrared light is directed across a planar area situated between the work zone of the machine and the operator's station to establish what is termed a light curtain. Photoelectric sensor means variously sound an alarm or stop operation of the tool if the light curtain is penetrated by an object as large or larger than the operator's fingers.
In some usages of a light curtain system, work stock being processed at the machine tool must be positioned and held in place by the operator during at least certain stages of the work cycle. This does not pose serious problems in situations where the work stock is sufficiently lengthy to extend outside of the region protected by the light curtain. Under those conditions, the work stock may be gripped by the operator without triggering the protective mechanisms. In such cases, either a thin space is present below the light curtain to allow for extension of the work stock toward the work zone or the sensitivity of the photoelectric sensor means is adjusted so that it is not triggered by a thin sheet of work stock but does react to thicker objects such as an operator's hand.
The light-curtain type of protective mechanism has not heretofore been suited to a situation where work stock which must be held by an operator is too short to extend outside of the protected region. Moreover, an essentially similar problem can occur under circumstances where the work stock is initially large enough to be gripped at a location outside of the protected region. For example, large powered shear machines are often used in manufacturing plants to cut small pieces out of flat sheet work stock. As a series of such cuts are made the operator pushes the end of the flat sheet material progressively closer to the light curtain. If the light curtain were to be situated extremely close to the cutting zone, the desired protective function may not be fully realized as a finite period of time is required to stop cutting blade movement. On the other hand, if it is distant from the work zone under the condition described above then eventually a stage is reached where the operator can no longer hold the end portion of the work stock without penetrating the light curtain with his hands. Under these circumstances it has heretofore been necessary to discard the terminal portion of the work stock although there may be sufficient material left to form one or more of the desired cut pieces. In a high-volume manufacturing operation, this can be a very significant source of material wastage.